Implantable medical devices, such as orthopedic and dental prostheses, can be made more permanent if the interface between the existing bone and the device contains some natural bone growth to knit the two components together. Such ingrowth has advantages over the use of bone cement, both in terms of stability and permanency.
“Bioactive” coatings on implantable medical devices allow for the ingrowth of natural bone into and around the device, forming chemical bonds between the device and natural bone. Bone is composed of substituted apatite crystals in an abundant collagen network. Type I collagen is the major protein of bone tissue, making up about thirty percent of the weight of bone. It has been shown that apatite crystals can grow and bond to collagen fibrils, and prepared apatite/collagen composites have been shown to promote direct bone apposition. However, there are drawbacks to these composites.
Electrophoresis has been used to prepare a bioactive apatite/collagen composite coating on a substrate. However, this method results in a relatively low bonding strength at the interface between the coating and the substrate.
Other groups have synthesized apatite/collagen composite coatings by the biomimetic method using simulated body fluid (“SBF”). The reported biomimetic methods took three days to obtain an apatite/collagen composite coating. The resulting coating contained collagen in colloidal/elliptical particles having sizes over two micrometers. Using a high saturated SBF solution (concentrated by a factor of five) containing collagen results in an inhomogeneous apatite/collagen composite coating which is unlike natural bone's ultra-structure at the nano-level. Under these conditions, the collagen fibers in the composite coating randomly overlapped and submicrometer apatite particles (200-600 nm) were attached on the collagen fibers.
Soluble collagen spontaneously forms into gel in neutral salt solutions within less than fours hours at physiological temperature (37° C.), while most of the apatite coating starts to deposit on substrate after fours hours under physiological conditions. As a result under traditional conditions in the biomimetic process, most of the collagen gelates and floats on the surface of the SBF solution or deposits on the surface of the substrate before the apatite precipitates. In the concentrated SBF case, apatite deposits on the surface of the collagen coating to form a laminated structure, and not a uniformly mixed composite. In both cases, only a small proportion of collagen is incorporated into the apatite coating and forms the nano-level apatite/collagen composite coating.
There remains a need in the art for improved bioactive composite coatings in addition to processes to prepare the composite coatings.